1. Field of the Invention
The invention relates generally to construction of lines in cartographic applications by software simulation and particularly to a method for digitally constructing minimum width and variable width lines via vector-to-raster conversion.
2. Description of the Prior Art
Generation of map images by automated equipment often requires that lines of differing widths be produced. Equipment commonly referred to as vector equipment provides this capability in an essentially mechanical fashion through the hardware employed and includes such options as pen selection on pen plotters or aperture selection on photoplotters. When utilizing graphic devices operable by binary raster output, these hardware options are not available, such equipment requiring the generation of line widths by software simulation. A primary difficulty encountered with such equipment in the specific field of cartographic symbolization is that lines generated by software simulation typically do not meet necessary standards. The deficiencies of the prior art become of even greater moment in view of recent developments in the use of output devices for vector data which involves the generation of lines in raster form using vector-to-raster conversion. Given such advances in the state of the art relative to the generation of cartographic lines in automated image generation, it has become imperative that the ability to control the quality of line widths generated by software simulation be improved.
The prior art as represented by issued United States patents does not provide such improvement with even those devices closest in function to the present problem being of little assistance in meeting cartographic requirements. In particular, Kemplin et al, in U.S. Pat. No. 4,135,245, describe an X-Y plotter having a programmable pen changer. However, the Kemplin et al device is not useful in the generation of variable width cartographic lines by software simulation. Rieger, in U.S. Pat. No. 4,283,765, teaches the use of a matrix multiplier for rotation, scaling, shearing, and stretching of three-dimensional caligraphic images with particular application to incremental vector generators. Andrews et al, in U.S. Pat. No. 4,259,725, disclose a system for displaying a rasters-scanned image on a screen wherein signals are also fed for causing a cursor having distinctive light intensity to be overlayed on the image. Andrews et al further disclose programs for defining multiple cursors which are stored in a readable memory. As a further example of the prior art, Hennessee, in U.S. Pat. No. 4,062,648, discloses a plotter system having an adaptive velocity device for improving plotted line quality by selectively varying the velocity of the plot head. However, the systems of the prior art do not provide the ability to construct variable-width cartographic lines through software simulation and which is of a quality adequate to the requirements of the generation of map images.
The present invention provides a technique for generating lines via vector-to-raster conversion wherein the lines have a minimum distinguishable width associated with cartographic production, the invention further allowing the generation of variable-width lines in addition to the generation of lines of minimum width.